Zeroing to the center of a bore

Hi everybody,

I'm a hobby CNC user with very little time for my hobby, so I go sometimes months between using my machine.  I Used to run Mach 3 and I converted to Linux CNC a while back.  I was trying to indicate to home my machine in the center of a large bore.  In Mach i'd zero one side of the bore, then touch the other side then key in /2 to get the center on that axis.  I could not seem to figure out how to do this in linux cnc, so I was with a calculator doing the division and that would work, but one time i screwed up and did'nt type in the - sign, then I issued the G0x0y0 and destroyed my Haimer.  Bad day!  Anyway, I'm not ready for fancy probing stuff yet.  What is the proper way to set the home position to the center of a bore using a edge finder?

Thanks